ject becomes more widely understood, the examples of any other method of treating fuel will become rare.

The use of gas as fuel is based on the principle that at one place all the fuel is converted into combustible gas, which is consumed by admixture with the necessary amount of air at another. These gases are produced by burning the fuel in a long column, whereby most of the carbon is burnt to carbonic oxide, whilst the hydrogen either remains free or is converted into carburetted hydrogen. Directly above the grate carbonic anhydride is formed, but this is converted into carbonic oxide on passing through the column of heated fuel above it. The reaction is shown by the following equation:-CO,+C=2CO. As a rule, the finer the fuel, that is, the more compactly it lies, the lower is the column required for the reduction of the carbonic anhydride. Besides the products of combustion mentioned above, distillation products are formed in the upper layers of the fuel and mix with the combustible gases. Amongst these, heavy carburetted hydrogen (C,H) is that which principally increases the value of the gas as fuel, and the drier the fuel used the more of this gas is produced.

The conversion of fuel into combustible gas is effected in special apparatus, termed producers, from which it is conducted to the furnace, and burnt. For the formation of producer-gases a certain temperature is required, which should not be exceeded. This temperature is dependent on the amount of air introduced. If this is large, complete combustion is effected, and the desired object is only imperfectly attained. If, however, it is too small, the heat is not sufficient for the formation of carbonic oxide, and the evolution of gas ceases. The exact amount of air that should be introduced into a gas-producer can only be determined by experiment.

The composition of producer-gases is, by weight, as follows:

The gas-producer first assumed importance in 1856, on the introduction of the Siemens regenerative system. In 1861 the well-known Siemens gas-producer was patented. It consists of a chamber lined with fire-brick, with one side sloping at

an angle of 45° to 60°, with the grate at the bottom. The grate thus resembles the step-grate largely used on the Continent. The fuel is charged in at the top of the incline, and falls in a thick bed upon the grate, where air is admitted. Passing from the top of a brick shaft or up-take, 8 to 10 feet high, placed above the producer at the back, there is a cooling tube, having not less than 60 square feet of surface per producer. Its object is to cool the gases issuing from the producer, thus giving them in reased density, causing an onward movement towards the furnace, and rendering it unnecessary to place the producer at a much lower level than that of the furnace. This cooling, however, results in a condensation of tar, and, to overcome this annoyance, modifications have been adopted in the producer and its working. In the new type of Siemens producer the volatile products of distillation are obliged to descend through highly heated fuel, thus causing the tar to undergo decomposition.

The principal forms of gas-producers are described by Rowan. Since the introduction of the regenerative principle, many designs for furnaces have been proposed, without separate gas-producers. One of the most successful of these is the Boëtius furnace, largely used for zinc-smelting.

The Wilson gas-producer (Fig. 52), working under a slight pressure, presents the advantage of burning fine slack coal. Fuel is introduced in the chamber A, through a hopper, h. The ashes are drawn by two small doors, c, at the bottom every twenty-four hours, the operation occupying about twenty minutes. During this time the production of gas is stopped. The producer represented in the figure has a diameter of 8 feet, and is constructed to burn 4 cwt. of small coal per hour. Air is injected into the chamber by two steam jets, b, each having a diameter of inch. Analyses of gas from the Wilson producer gave the following results:

I. is from a producer using Durham coal. II. is an average of six samples of gas taken over a time of one hour from a producer working on fine slack from a Yorkshire colliery. III. is from a producer working with coal of the Jemmapes district, near Mons, Belgium.

In 1814 Aubertot first used the waste gases of blast furnaces for roasting ore, burning lime, and similar purposes, and these

gases are now largely used when a very high and uniform temperature is not required. The composition by weight of the waste gases, according to the fuel used in the blast furnace, is as follows:

Water-gas.-The gaseous fuel known as water-gas is made in the following manner:-An iron cylinder is lined with fire-brick, and provided with the necessary apparatus for introducing the coke. When this has been lighted, a current of air is forced in until the mass is brought to a high temperature. The blast is then stopped, the charging aperture is closed, and a jet of steam is passed through the incandescent carbon. The steam is decomposed; its oxygen burns the carbon into carbonic oxide, setting free the hydrogen. The resulting mixture is known as water-gas. It consists of one volume of hydrogen and one volume of carbonic oxide, the weights being in the proportion of 2 to 28. If, by burning one unit of carbon, it were possible to generate one unit of hydrogen, the exchange effected in the water-gas apparatus might be a very profitable one. Such a condition of things is, however, shown by Sir Lowthian Bell to be directly opposed to the known facts of the case, for 25 per cent. only of the carbon used is burnt to the condition of water-gas, whilst the other 75 per cent. is converted into producer-gas, containing 68 per cent. of inert nitrogen. From 25 parts by weight of carbon there will be generated 62.5 parts of water-gas, containing 4.16 of hydrogen and 58.34 of carbonic oxide. The producer-gas from the remainder (75 parts) of the carbon will weigh 551.19 parts, of which 376.19 will be incombustible nitrogen and 175 carbonic oxide. The following estimate gives the quantity of heat generated by the combustion of the two gases:

Had the 100 parts of carbon been burnt direct, the heat generated would have been 800,000 calories. The loss is thus 14.71 per cent. Besides this, as coke was used, there is the loss of combustible matter which is incurred at the coke-oven, and the labour in conducting the process of coking.

Sir Lowthian Bell calculates that the relative values of coal, producer-gas, and water-gas are as follow:

Bibliography.-For fuller information on fuel the student is referred to the following standard treatises :-Percy, Metallurgy, vol. i. London, 1861; second edition, 1875; Galloway, Treatise on Fuel, London, 1880; Schwackhöfer and Browne, Fuel and Water, London, 1885; Phillips and Bauerman, Elements of Metallurgy, London, 1887, pp. 16-107; Mills and Rowan, Fuel and its Applications, London, 1889; Gruner, Traité de Métallurgie, Paris, 1875, vol. i. pp. 36-161; Kerl, Grundriss der allgemeinen Hüttenkunde, Leipzig, 1879, pp. 36-198; Muck, Grundzüge der Steinkohlen-Chemie, Bonn, 1881; Fischer, Chemische Technologie der Brennstoffe, Leipzig, 1887. Schnabel, Lehrbuch der Allgemeinen Hii'tenkunde, Berlin, 1890.

Much information is also to be found in the Journal of the Iron and Steel Institute, 1892. A. Naumann: Technisch-Thermochemische Berechmengen zur Heizung, Brunswick, 1893: For the use of students, the series of calculations contained in this work is of much value. The author shows the number of calories liberated by the combustion of the chief constituents of coal or producer gas, and gives the specific heats of the combustion products; he next sets a series of questions relating mainly to the combustion of such gas, or of coal, and then gives in each instance the calculations necessary to obtain the answers to the questions set.